Large amounts of carbohydrates in plant biomass provide a plentiful source of potential energy in the form of sugars (both five carbon and six carbon sugars) that could be utilized for numerous industrial and agricultural processes. However, the enormous energy potential of these carbohydrates is currently under-utilized because the sugars are locked in complex polymers, and hence are not readily accessible for fermentation. These complex polymers are often referred to collectively as lignocellulose. Sugars generated from degradation of plant biomass could provide plentiful, economically competitive feedstocks for fermentation into chemicals, plastics, and fuels, including ethanol as a substitute for petroleum.
For example, distillers' dried grains (DDG) are lignocellulosic byproducts of the corn dry milling process. Milled whole corn kernels are treated with amylases to liquefy the starch within the kernels and hydrolyze it to glucose. The glucose so produced is then fermented in a second step to ethanol. The residual solids after the ethanol fermentation and distillation are centrifuged and dried, and the resulting product is DDG, which is used as an animal feed stock. Although DDG composition can vary, a typical composition for DDG is: 32% hemicellulose, 22% cellulose, 30% protein, 10% lipids, 4% residual starch, and 4% inorganics. In theory, the cellulose and hemicellulose fractions, comprising about 54% of the weight of the DDG, can be efficiently hydrolyzed to fermentable sugars by enzymes; however, it has been found that the carbohydrates comprising lignocellulosic materials in DDG are more difficult to digest. To date, the efficiency of hydrolysis of these (hemi) cellulosic polymers by enzymes is much lower than the hydrolytic efficiency of starch, due to the more complex and recalcitrant nature of these substrates. Accordingly, the cost of producing the requisite enzymes is higher than the cost of producing amylases for starch hydrolysis. It is therefore desirable to produce inexpensive enzyme mixtures that efficiently degrade cellulose and hemicellulose.